A photovoltaic power generation system is usually composed of three parts: a photovoltaic panel array, a direct-current boost converter and a grid-connected power converter. The direct-current boost converter and the grid-connected power converter are interconnected and isolated by an intermediate direct-current bus, and are usually as a whole, called a photovoltaic inverter. The main working principle is that the direct-current boost converter boosts low-voltage direct current output by the photovoltaic panel array to high-voltage stable direct current for converting into alternating current by the back stage grid-connected power converter and then feeding into the power grid. With the increase in grid-connected power of photovoltaic power generation systems and the dropping of the maximum power point (MPP) of photovoltaic panels (PVs), single-channel direct-current boost circuits are difficult to take account of application scenarios of both high power and high boost ratio. Thus use of a multi-channel MPPT input to improve grid-connected power generation of photovoltaic inverters is a research hotspot of various manufacturers.
Without loss of generality, using the more applied dual-channel MPPT input photovoltaic inverter as an example, the existing multi-channel MPPT control logic is described. FIG. 1 shows a structural diagram of a dual-channel PV input photovoltaic inverter system 10 comprising photovoltaic panels PV1, PV2, input capacitors C1, C2, input boost circuit boost1 12, boost2 14, a direct-current bus capacitor Cdo, an H-bridge inverter 16 and a controller DSC 18. Outputs of the boost circuit boost1 12 and boost2 14 are connected to a common direct-current bus 20, and power is fed into the power grid 22 through the bus capacitor Cdo and the H-bridge inverter 16. PWM1 and PWM2 are drive signals of the boost circuits boost1 12 and boost2 14, respectively. The controller DSC 18 generally implements dual-channel MPPT control using a digital signal processor (DSP) by acquisition of information of PV input voltages vPV1 and vPV2 in two channels, PV input currents iPV1 and iPV2 in the two channels and a direct-current bus voltage vbus, a brief control flow chart 40 is shown in FIG. 2. In FIG. 2, vPV1*, vPV2* and vbus* are reference signals of the PV voltages in the two channels and direct-current bus voltage, respectively, ΔvPV is the difference between the PV voltages in the two channels, ΔvPV=vPV1−vPV2; VTH is a judging threshold of the difference between the PV voltages in the two channels. The controller DSC obtains PV input power signals PPV1 and PPV2 in the two channels by sampling the PV input voltages vPV1, and vPV2 in the two channels at 44 and the PV input currents iPV1 and iPV2 in the two channels at 42, and obtains the PV voltage reference signals vPV1* and vPV2* through their respective MPPT module operation. At the same time, the controller DSC calculates the difference ΔvPV between the PV voltages in the two channels at 46, and substitutes it into a boost start and stop control logic to compare with the preset threshold VTH. There are three cases:
ΔvPV≥VTH (Y at 48), the controller DSC turns off a boost1 controller, blocks the drive signal PWM1 of the boost1 circuit, turns off the boost1 circuit at 50, enables a boost2 controller and obtains the drive signal PWM2 of the boost2 circuit, and the direct-current bus voltage reference signal vbus*=vPV1* at 52.
ΔvPV≤VTH (Y at 54), the controller DSC turns off the boost2 controller, blocks the drive signal PWM2 of the boost2 circuit, turns off the boost2 circuit at 56, enables the boost2 controller and obtains the drive signal PWM1 of the boost1 circuit, and the direct-current bus voltage reference signal vbus*=vPV2* at 58.
VTH≥ΔvPV≥−VTH, (N at 54) the controller DSC enables both the boost1 and boost2 controllers and obtains the drive signals PWM1 and PWM2 of the boost1 and boost2 circuits at 60, and the direct-current bus voltage reference signal vbus* uses the maximum of the two PV voltage reference signals, i.e., vbus*=max(vPV1*, vPV2*) at 62
In the practical application, comprehensively considering thermal balance of a direct-current boost circuit of the inverter, conversion efficiency of the whole inverter and the service life of components, for the multi-channel MPPT input inverter, photovoltaic panels in all channels are usually configured almost uniformly, thus a PV curve of each input of the inverter is approximately the same. It can be seen from FIG. 2, using the existing multi-channel MPPT control method, each boost circuit is in a working state at a steady operating point, the conversion efficiency of the inverter is low, and the grid-connected power generation is small.